Thank you so much to all, especially to turbo1889. It seems the rechargeable batteries have dangers during not only the charging but also discharging (over-discharging) process, either explosion or damage to the battery. If I don't frequently ride at night, the rechargeable batteries will be left alone for weeks or months. Will that be dangerous, too?

P.S. is 26650 more powerful than 18650? because it's a larger number? (obviously this is a dumb newbie asking )